The present invention relates to process control transmitters used to measure process variables in industrial processing plants. More particularly, the present invention relates to a process control transmitter having an externally accessible DC circuit common.
Process control transmitters are used in industrial processing plants to monitor process variables and control industrial processes. Process control transmitters are generally remotely located from a control room and are coupled to process control circuitry in the control room by a process control loop. The process control loop can be a 4-20 mA current loop that powers the process control transmitter and provides a communication link between the process control transmitter and the process control circuitry. Typically, the transmitter senses a characteristic or process variable, such as pressure, temperature, flow, pH, turbidity, level, or the process variables, and transmits an output that is proportional to the process variable being sensed to a remote location over a plant communication bus. The plant communication bus can use a 4-20 mA analog current loop or a digitally encoded serial protocol such as HART(copyright) or FOUNDATION(trademark) fieldbus protocols, for example.
Referring now to FIG. 1, a simplified block diagram of a process control transmitter as can be found in the prior art is shown. Here, process control transmitter 10 includes housing 12, circuitry 14, and first and second terminals 16A and 16B. Housing 12 is not permanently hermetically sealed and generally includes lower housing member 12A and removable cap 12B. A seal (not shown) is typically sandwiched between lower housing member 12A and cap 12B to seal housing 12. Process control loop 18 can couple process control transmitter 10 to control room 20 at first and second terminals 16A and 16B. Circuitry 14 is configured to receive a sensor input 22 relating to a process variable and communicate the process variable information to control room 20 over process control loop 18.
Circuitry 14 generally communicates with control room 20 over process control loop 18 by adjusting loop current IT flowing through process control loop 18 and first and second terminal 16A and 16B. Circuitry 14 senses loop current IT with feedback output FB, which relates to the voltage at node 24 with respect to DC common 26 or the voltage drop across sense resistor RSENSE. Feedback output FB is communicated to circuitry 14 through conductor 28 which includes series resistor RSERIES which allows a negligible amount of current to flow through conductor 28 between node 24 and circuitry 14. Circuitry 14 uses feedback output FB to adjust loop current IT in accordance with the sensor input 22.
The voltage drop across sense resistor RSENSE, second terminal 16B has a voltage that is offset from DC circuit common 26 by the voltage drop across RSENSE. Additionally, the voltage difference between second terminal 16B and DC circuit common 26 will vary as loop current IT is varied by circuitry 14. As a result, communication signals produced by circuitry 14, which are regulated with respect to DC circuit common 26, cannot be conveniently communicated to processing circuitry that is external to process control transmitter 10 without performing a level shift in the voltage of the communication signals to compensate for the voltage drop across sense resistor RSENSE. This level-shifting requirement would result in increased cost and complexity of processing electronics that are to be coupled to transmitter 10 and adapted to communicate with circuitry 14 using signals which are regulated with respect to DC circuit common 26. Additionally, there is an increase in the potential for error due to mismatched level-shifting or DC circuit common.
A process control transmitter having an externally accessible DC circuit common is provided that eliminates the need to perform level shifting of signals communicated between the transmitter and external processing electronics. The process control transmitter includes first, second and third externally accessible terminals, a series regulator, circuitry, a shunt, and a shunt current regulator. The first and second terminals are coupleable to a process control loop and are adapted to conduct a loop current through the transmitter. The circuitry is energized by a load current and is generally adapted to manage process variable and transmitter-related information and provide a digital signal to the third terminal that is regulated relative to a DC circuit common. The DC circuit common is electrically coupled to the second terminal and the digital signal is externally accessible between the second and third terminals. The series regulator is coupled to the first terminal and is adapted to conduct the load current and provide a first feedback output that is representative of the load current. The shunt is adapted to conduct a shunt current and provide a second feedback output that is representative of the shunt current. The loop current is substantially a summation of the load current and the shunt current. The shunt current regulator carries the shunt current and controls the loop current as a function of the first and second feedback outputs.